A groving tree as a woody perennial plant contains relatively huge amounts of moisture content warying from one kind to another which stays in the tree after being cut down. As it is known wood contains capillaries in cells that contain free liquid and absorbed moleculas of water called moisture content which must be lowered to a certain level to satisfy needs of industrial use. For that reason, wood has to be properly treated, namely dried to a certain value of acceptable end moisture distribution content in a way to prevent occuring of all kinds of drying defects. The drying process is a mayor factor in economic terms.
Wood could be dried under natural circumstances in the open-air drying if wood climate relation conditions are good enough and consequently generating acceptable quality of dried sawn wood. It has to be considered that air drying of wood is a long term process which could extend into years. When finally wood is dried it needs proper storage conditions that include natural circulation of dry warm air and other terms of planning. It has to be considered that absorbed water in the wood can emerge and evapoprate with the help of sourrounding air flow only in case if the surface is not covered by rain fall water, snow or other substances. Influencing on air drying tehnology by restacking with ventilation abilities do help in minor values with the constant risk of wood being attacked by mould, microoorganismus, fungi insects including uneven drying that can worsen quality of wood expresed with other terms concerning its quality. There is always a potential problem of shape deformations that can emerge because of natural air-drying unpredictable situations that cannot be prevented by any preventive process control tehnology or monitoring of any kind. Moisture distribution content in wood is by using technology of drying by air after a certain period of time depending on climate conditions what means that time variation is present in planning emerging other problems usually leading to higher costs, too much rejections of quality assurance and alike. On the other hand there are also some other efects of such a technology like low energy consumption, huge drying areas, storage departments, safety procotions. There were some expriments executed to shorten air-drying process by engaging axial fans, but there are too many other parametres like relative humidity and others on which axial fans cannot influence.
With the intention to reduce drying time in the aspect of cost, quality and time were developed drying devices of different kinds, which can be distinguished by a technological approach in a following manner: The first type is a compartment type and tunnel kiln type. Both known types of drying devices can be characterized as stationary types. By the first type of drying devices the wood is placed in avaliable compartment which has the ability of generating different physical conditions like: temperature, humidity, air flow capacity and alike, with the intention of proceeding of the drying process. By the second type of drying devices the wood is transported with the help of horizontal transport unit through the drying device where it is sequently processed under different physical conditions, mainly for the purposes of gradualy executing the drying process. Compartment type drying devices are cheaper, but with lower production rate in comparison with the tunnel kiln type drying devices, where investment costs are relatively high.
The already known types of drying devices have certain disadvantages which will be explained in details as follows including the decisions of great investment cost and setting up difficulties in aspect of economy factors.
The technology processes used nowadays by drying the wood are performed either by low temperatures between 15 and 45° C. or by medium temperatures between 45 and 90° C. or also by high temperatures between 90 and 130° C. with the posibility of achieving above specified temperatures e.g. by means dielectrical, convectional, conduction or radiation principles.
By certain types of drying devices the wood is put into compartment by means of suitable transport carriages. With the ventilators placed on the ceiling or rarely on other locations an air flow is created which is in some cases blown transversely, yet by others the air flow is lead horizontaly and transversely and still in other versions the air is lead longitudinaly.
By all these known drying devices the transport units are constructed in a manner and with such dimensions that enable loading as great quantities of wood as possible in the kiln volume. The wood is stacked by along ventilation in a way that air flow is possible at least in one horizontal plane. A certain compromise has to be achieved with the consideration of dimensions of the air gap that is neccessary for air flow and the amount of wood in the kiln. The hot air is then blown through the air gaps in order to fasten up the intensity of drying. When the hot air gets in contact with the wood containing high moisture level it absorbs it to its highest possible value, what causes enabling of absorption of moisture, that is still present deeper in the pile. Consequently that means that ventilators create enough strong air flow yet with the highest moisture level possible what means only a lot of waste of energy. Because of high moisture level in the air it is very likely that it condenses on cooler places such as walls and other equipment causing damage. The condensed liquid that stays in the kiln volume efects harmfull on it as well as to the drying process.
As noted in the patent aplication EP 0 170 648 A1 which is intended to execute one of the latest drying technologies the compartment has warm-insulated walls. The sawn wood is being stacked by longitudinaly ventilation in the kiln volume. In the drying device there is installed a ventilator which enables air flow passing through a heating register then continuing on trough stacked wood to the cooling register where the air flow is led in a way of repeating the same loop. In the area of heating register the air is warmed up then as passing trough the stacked wood it picks up moisture which is then released by passing trough the colling register to the warming register. Such a combination is likely to create condensate if fresh sawn wood is processed but is quite suitable for wood with low moisture volume—the final touch—before being used up by industry.
If desired that by means of satured air generated by drying process of stacked wood as highest as possible quantity of moisture should be departed from compartment, the air must be heated. The disposal of great amounts of saturated air is combined with great looses of heat used before as heating air. Energy yield by using this technology of drying wood is low.
Except of the above mentioned drying technologies also a vacuum drying technology is described in the PCT/DK87/00012 and WO 87/04779, where e.g. intensivity of drying process could be monitored in order to avoid drying defects. The devices with applied vacuum technology are very sensitive in mantaining proper vacuum conditions and are more suitable for drying processes for wood containing lower values of moisture what means that other drying technology for eliminating the majority of moisture has to be used therebefore.
By all these known solutions it can be summarized that all of them have certain imperfections, e.g. relatively low energy yield being unacceptable for global economy, or high requirements in respect of the space consumption, highly dependance on power sources, a high probability of drying defects, very small or no adaptable abilities and are moreover built as a stationary type with drying capacities that dictate the amounts of drying wood and technology.